1. Field of the Invention
The invention is a blood reservoir with at least one pliable wall having at least three innovative features. First, the compliant reservoir is sealed within a rigid housing allowing control of the “atmospheric” pressure surrounding the bag, and therefore the pressure at which the bag would collapse. This first invention enables vacuum augmented venous drainage (VAVD) with a collapsible soft-shell reservoir (i.e. venous bag) and is particularly useful for cardiopulmonary bypass. Second, the invention incorporates means that improve gas bubble removal from blood transiting the collapsible reservoir. Third, a reservoir with all ports extending from its top is disclosed, an innovation that provides easy loading/unloading of the reservoir in/out of its holder and simplifies sealing the reservoir in a chamber thereby allowing the aforementioned VAVD.
2. Description of the Prior Art
Blood is routinely pumped outside the body during dialysis, cardiopulmonary bypass, and long-term cardiac and/or respiratory support (e.g. extracorporeal membrane oxygenation, ECMO). In general, blood flows from the venous side of the patient to a venous reservoir that is usually maintained at atmospheric pressure. Blood flow from the patient to the reservoir is a function of the resistance of the fluid conduit between patient and reservoir, and the pressure difference between patient and reservoir. When the reservoir is maintained at atmospheric pressure, that pressure difference is the height difference between patient and reservoir; the resulting flow is referred to as gravity drainage. Venous drainage by gravity alone provides inadequate return during procedures such as minimally invasive cardiac surgery and bypass via femoral cannulation. Usually it is the resistance of the venous cannula that limits the flow rate. Vacuum augmented venous drainage (VAVD) is a technique that overcomes flow limitations by applying suction to the hard shell reservoir thereby increasing the pressure difference between the venous cannulation site and venous reservoir. VAVD allows for a decrease in the inner diameter of the venous line, thereby reducing prime volume and enabling the use of a smaller cannula, which translates to an easier insertion, a better surgical view and a smaller surgical incision. This method precludes the use of the safer soft-shell closed venous reservoir (venous bag) unless a more expensive and complicated two-pump system is used (see McKusker K, Hoffman D, Maldarelli W, Toplitz S, and Sisto D. High-flow femoro-femoral bypass utilizing small cannulae and a centrifugal pump on the venous side, see Perfusion 1992; 7:295–300.
Clinically, a venous bag is used because it provides significant safety features. If the bag empties, it collapses, thereby preventing gross air from being pumped to the patient. It usually has no air-blood interface, and it requires no antifoam agents that can embolize into the blood. A recent study by Schonberger et al (Schonberger J P A M, Everts P A M, and Hoffmann J J. “Systemic blood activation with open and closed venous reservoirs. Annals of Thoracic Surgery, 1995; Vol. 59, pages 1549–55) comparing the hard shell to the bag reservoir found significantly lower blood activation, shed blood loss, crystalloid infusion, and hemolysis, and less donor blood infusion with the bag reservoir. Schonberger's group recommended against routine use of an open (hard shell) venous reservoir system. Currently, a slight negative pressure applied to the venous line (to facilitate blood drainage) using a single pump is possible with less desirable hard shell venous reservoirs. It is impossible to apply negative pressure to current soft-shell reservoirs, but it is possible with the present invention.
In an open, hard shell reservoir, air escapes by floating to the top of the reservoir. In a bag reservoir, air floats to the top but must be actively eliminated. This can be done manually with a syringe, or more frequently with a roller pump operating slowly so as to continuously pump fluid to the cardiotomy reservoir. With either method, a sudden large volume of air can overwhelm the air removal system and cause disastrous consequences, especially without a vigilant perfusionist. With one preferred embodiment of the present invention, air would be eliminated automatically without a roller pump or intervention by the perfusionist, and priming of the extracorporeal circuit would be facilitated through faster air removal utilizing either a floating ball valve or a hydrophobic membrane. Currently there are devices used in the CPB circuit that incorporate hydrophobic membranes that remove air yet do not allow blood to cross (e.g. Model # AutoVent-SV, Pall Corp Glen Cove N.Y.). Studies with filters used in these applications have shown that the membranes clear air from water almost indefinitely (many days), even if high suction is applied, without reducing gas transfer rate over time. However, when the membrane is exposed to blood, especially when high suction is applied, a film forms on the membrane over time, causing a significant increase in resistance to gas flow. The present invention incorporates designs and means to reduce this problem and extend the life of the membrane when used with blood. Likewise, U.S. Pat. No. 3,849,071 shows a floating ball within a blood filter that supposed to open a purge port when air enters and close when the blood level rises. However, as described, it is a physical impossibility for the ball to “fall” and open the purge port because, as shown, the weight of the floating ball is insufficient to overcome the force holding the ball against the purge port. With the present invention, the relative weight of the ball, the internal diameter of the purge port, and the suction applied to the purge port are designed to assure that the ball will drop to open the purge port in response to air level in the venous reservoir.
With prior art soft-shell reservoirs (SSR) air may be trapped at the top of the liquid by the collapsed walls of the reservoir, see FIG. 1a and 1aa. U.S. Pat. No. 4,622,032 illustrates a soft shell reservoir having an inlet tube extending from the bottom half way into the reservoir. This arrangement helps bubbles move up to the top of the extended tubes but the bubbles can still be trapped above said tubes. U.S. Pat. No. 5,573,526 illustrates the prior art soft-shell reservoir having its gas removal tubes (i.e. 18 and 20 of FIG. 1) extending from the top less 40% of the height of its blood chamber into the reservoir. All other prior art SSR have air removal tubes that are shorter with many having vent tubes that do not extend into the SSR at all (e.g. U.S. Pat. No. 5,580,349). As FIGS. 1b and 1bb illustrate, a tube extending from the top and into the SSR prevents complete collapse of the pliable walls of the bag thereby forming a pathway for air to move upward. The prior SSR air removal tubes extend less than 40% of the height of the blood chamber and therefore air still may be trapped below said tubes.
A soft shell venous reservoir sold by Medtronic Model # Maxima 1386 shows a soft shell reservoir with an inline tube extending, along one side of the bag, to the gas purge port at a 45° incline. This design has a rigid fluid path between blood inlet and gas purge port. However, this design is not as conducive to air removal as a vertical fluid path would be. In addition, the tube extending between inlet tube and purge port had an ID of ⅝″, or only 25% greater diameter than the inlet tube. Thus, the velocity of the liquid in the column slows to only 64% of the inlet velocity. In another aspect of the present invention, a vertical path is provided from the blood inlet at the bottom of the bag to the gas purge port at the top of the bag, such path limiting the aforementioned problem of trapped air. The vertical path also has a large enough diameter that slows the velocity of the liquid to 25% or less of the inlet velocity. A lower blood velocity is more conducive to bubble removal.
State of the art soft shell venous reservoirs with a screen are designed such that a large portion of the screen contacts the internal walls of the bag, thereby increasing the resistance to blood flow across the screen, and rendering that portion of the screen ineffective, at least partially. This contact between the screen and the walls of the bag increases as the volume in the reservoir decreases. One aspect of the present invention reduces that problem by preventing the external walls of the venous reservoir from contacting the screen.
The indication of blood level in present soft shell venous reservoir is very inaccurate and low level, or air-in-the-reservoir, alarms are not reliable because many are designed for hard shell reservoirs. The present invention provides effective means to alarm at low blood levels and in the soft shell venous reservoir.
Currently, at the end of the bypass procedure, the patient is weaned off the heart lung machine by reducing the bypass flow. This is achieved by partially clamping the venous line and decreasing the speed of the arterial pump. Once off bypass, the blood left in the venous reservoir is gradually pumped back to the venous side of the patient. Another aspect of the invention allows the user to adjust the positive pressure applied to the blood within the venous reservoir. By being able to increase the pressure of the venous reservoir, the user can effectively reduce venous drainage or perfuse the blood back to the patient. This is not possible with current venous reservoir bags and may be dangerous with hard shell reservoirs (i.e., air may be pushed to the patient).
The inventor has also previously described an inline bladder (The Better-Bladder™, now U.S. Pat. No. 6,039,078), a device with a thin walled, sausage shaped bladder sealed inside a clear, rigid housing. Since the bladder is made from a single piece of tubing, the blood path is smooth with no flow discontinuities. The bladder portion is sealed within the housing that has an access port to the housing space outside the bladder. Because of its thin wall, the enlarged section can easily collapse. Thus, it can serve as an inline reservoir, providing compliance in the venous line to reduce the pressure pulsations at the pump inlet. The Better-Bladder also transmits the blood pressure flowing through it across its thin wall, allowing pump inlet pressure to be measured noninvasively by measuring the gas pressure of the housing via the gas port. The degree of “gravity drainage” is user-adjustable by setting the negative pressure in the Better-Bladder housing. If the suction generated by the venous pump becomes too great, the pump is slowed or stopped by a pump controller. The Better-Bladder does not have a gas purge port or a screen to inhibit gas bubbles. It is also much smaller, having nominal volume of 80 ml for adult perfusion as compared to over 1,000 ml for a venous reservoir.
Despite users acknowledgement that SSR are safer, hard shell reservoirs are easier to use and therefore more widely used. For example, it is easier to connect the inlet tubing located at the top of the hard shell reservoir than to the inlet tube of the SSR located at the bottom. Though some SSRs are premounted by the manufacture to a supporting plate (e.g. Cobe see U.S. Pat. No. 5,693,039), most require multiple hanging hooks for proper support (e.g. Baxter's SSR model # BMR1900 has 3 holes at the top and 4 holes at the bottom), an inconvenience at best, a danger in an emergency. Present mounted SSR do not improve the tube connection by much. It would be a clinical advantage to provide a SSR that allows fast mounting and dismounting, and tube connection that are easy or even easier than that of hard shell reservoir. Another requirement for present SSR is the use of a supporting faceplate (e.g. see FIG. 3 of U.S. Pat. No. 5,573,526). These are used to improve the bubble path from the blood to the top of the reservoir. Such faceplates are again inconvenient, require additional assembly time by the user, and may obstruct the direct approach to the front wall of the bag. The latter is useful when bubbles “stuck” on the wall are to be dislodged. The elimination, or at least the reduced requirement, of a front plate is another desirable attribute.
U.S. Pat. No. 5,823,045 “Measuring Blood Volume in Soft-Shell Venous Resevoirs (sp.) by Displacement” illustrates a SSR enclosed in a rigid housing. This invention suggests sealing a SSR within a rigid housing but does not suggest applying vacuum to the fluid surrounding the SSR. In fact, neither the FIGs. nor the specifications mention a port for adjusting the fluid in sealed container 12. Van Driel's patent has some major flaws. The tubes connected to the bag are to be “threaded through resilient seals 26 in the bottom of container 12 . . . ” also renders '045 clinically irrelevant. If, though not described as such, container 12 is disposable, then the system as described is too expensive. If, as understood, container 12 were not disposable, then “threading” the tubes would break sterility, and would be very difficult, especially if a seal is required. Further, since the outside diameter of perfusion connectors are larger than the OD of the tubing they connect, it would be impossible to have any of the tubing of the SSR connected to anything until after they have been threaded. In addition, the housing needs to be sufficiently wide for the user to place their hands and thread the inlet and outlet tubes at the bottom of box 12, a major disadvantage that hinders quick setup and increases the likelihood of contamination. This invention has not been reduced to clinical practice.
Since the SSR is sealed in container 12, external means are provided by '045 to “massage” the SSR with “vibrator” 36. In fact, since vibrator 36 is not connected to the wall of the SSR, it can only squeeze the wall rather than the pull and push motions required for vibration. This provides significantly less manipulation ability as compared to direct contact with the bag.
PCT's International Publication Number WO 99/08734 entitled “System and Method for Minimally Invasive Surgery Vacuum-Assisted Venous Drainage” illustrates in FIG. 9 a standard SSR (“preferably BMR-800 or BMR-1900”) completely sealed in a rigid housing. It is also suggested that “ . . . a pressure differential between the interior and exterior of the reservoir . . . ” be maintained. This design is as impractical as that of US Pat, '045. If the bag and rigid housing are assembled and shipped to the end user as a single unit, the unit becomes very expensive and therefore would be used only for VADV cases. The expense arises from a housing required to support a large force. The force can be calculated as (Pressure)*(Area). Thus, to support a pressure of −250 mmHg with a safety factor of 2 for a box that holds a bag like the BMR-1900 (10″ high by 12″ wide), the force on each faceplate is 1200 lb! The inventors did not suggest, nor showed or described, a mechanism for the user to seal the bag in the box. Even if there was a mechanism, it would be very difficult, time consuming, and, as described with relation to '045, most likely to break sterility. And, once sealed it would be impossible for the user to contact the bag.
Both '045 and '08734 illustrate the rigid housing as a rectangular box. A better design to support the large external force due to the large area and vacuum used would be an ellipsoid cross section or at least rounded corners.
Both '045 and '08734 illustrate the great need for designs that allow simple, inexpensive, and quick means to seal and remove the bag from its container even with long tubing or large connectors without affecting its sterility. Simple and quick means to reach the enclosed bag would also be welcomed. The present invention overcomes these clinically non-workable prior art designs.
It is standard practice to place the venous reservoir above the oxygenator to assure that the microporous membrane is always under positive pressure. A negative pressure would result in air crossing the membrane and entering the arterial line, a very dangerous situation. When VAVD is used, suction can be applied to the venous reservoir only once the arterial pump is generating a positive pressure in the arterial line. Otherwise, the suction applied to the venous reservoir can draw air across the membrane. A one-way valve at the pump outlet prevents vacuum applied to the venous reservoir from reaching the membrane oxygenator, but a one-way valve incorporated into the outlet of the present venous reservoir is preferable. Means to assure that the gas side of the membrane oxygenator is always positive relative to the blood side would also be a major safety feature for all VAVD applications.